In fields of optical communication art, stereoscopic image display art, etc., light modulation devices capable of realizing a high-speed change of a light path and a high-speed open-close (on-off) operation of a light path have been desired.
The technical background of a light modulation device of the present invention for use in the field of optical communication art is described below.
In the field of optical communication art, optical switching devices are used for on-off of optical signals transmitted on the optical transmission line such as an optical fiber or a thin film optical waveguide, or used for a change of the optical transmission direction.
Conventionally used in the field of optical communication art is a mechanical optical switching system, for example, a system wherein an optical fiber is mechanically moved to change its connecting portion to other optical fiber or a system wherein an optical member such as a prism, a mirror or a lens is moved to change the light path.
In the mechanical optical switching system, however, the switching time is slow, e.g., several msec (millisecond), and moreover there is involved a problem of wearing properties in the driving part, and hence the switching device is deteriorated in the reliability.
On that account, a non-mechanical optical switching system wherein the optical path is changed by utilizing electrooptic (EO) effect, acoustooptic (AO) effect, magnetooptic (MO) effect, etc. has been recently proposed in place of the above-mentioned mechanical optical switching system.
As the non-mechanical optical switching system, there can be mentioned, for example, an optical switching system utilizing the EO effect of a nematic liquid crystal material as described in R. E. Wagner & J. Cheng, "Applied Physics", Vol. 19, 2921 (1980).
In FIGS. 16a and 16b, a liquid crystal optical switching device (1') proposed by R. E. Wagner is shown, and the optical switching device of FIG. 16a and the optical switching device of FIG. 16b are different from each other in the output direction of a light (2).
In the liquid crystal optical switching device (1'), both of a polarization beam splitter (31, 31) and a total reflection mirror (3, 3) are arranged before and behind a liquid crystal cell (11'), and the liquid crystal cell (11') is connected with a voltage signal input means which is not shown in FIGS. 16a and 16b.
In the liquid crystal cell (11'), a nematic liquid crystal material is contained. When a voltage is not applied to the liquid crystal cell (11'), the polarization direction of the light (2) transmitted through the liquid crystal cell (11') is made to be changed by 90.degree..
In the optical switching device (1') shown in FIGS. 16a and 16b, the incident light released from a laser or the like is split into a vertically polarized light () and a horizontally polarized light (*) by means of a polarization beam splitter (31) positioned on the light incidence side. Then, the the horizontally polarized light (*) is reflected by the total reflection mirror (3) on the light incidence side at right angles and transmitted by the liquid crystal cell (11'). On the other hand, the vertically polarized light () is transmitted by the liquid crystal cell and then reflected by the total reflection mirror (3) on the light outgoing side at right angles.
As shown in FIG. 16a, when an electric field is not applied to the liquid crystal cell (11'), each of the horizontally polarized light component (*) and the vertically polarized light component () of the incident light transmitted through the liquid crystal cell (11') is rotated by 90.degree. with respect to the polarization plane in accordance with twist of the orientation of the liquid crystal molecules, and the vertically polarized light component () and the horizontally polarized light component (*) are together released as output A (output intensity IA) from the polarization beam splitter (31) on the light outgoing side. When an electric field is applied to the liquid crystal cell (11'), the nematic liquid crystal molecules are orientated in the vertical direction against the substrate, and the polarized light of the incident light is transmitted through the liquid crystal cell (11') without any change. The transmitted light is released as output B (output intensity IB) from the polarization beam splitter (31) on the light outgoing side.
In the optical switching device (1'), as described above, the polarization plane of the light transmitted through the liquid crystal cell (11') in the case where a voltage is not applied to the liquid crystal cell (11') differs by 90.degree. from that in the case where a voltage is applied to the liquid crystal cell (11') so as to change the orientation of the direction of the nematic liquid crystal molecules. Utilizing this difference in the polarization plane of the light transmitted through the liquid crystal cell, change of the light path, namely, optical switching, is carried out.
In the above optical switching, when a voltage is applied to the liquid crystal cell (11') so as to change the orientation of the direction of the nematic liquid crystal molecules contained in the liquid crystal cell (11'), it is preferable that a light having the same intensity as the light beam (2), is released from the polarization beam splitter (31) on the light outgoing side in the direction of the above-mentioned output B, but practically, leak of a light takes place in the direction of the output A.
Also when a voltage is not applied to the liquid crystal cell (11'), leak of a light takes place in the direction of the output B from the polarization beam splitter (31) on the exit side. As a result, a light having a light intensity IA is released in the direction of the output A and a light having a light intensity IB is released in the direction of the output B from the polarization beam splitter (31) on the light exit side.
A crosstalk value (CR), which indicates optical switching performance, can be represented by the following formula using the output light intensities IA(off) and IB(off) given when a voltage is not applied to the liquid crystal cell. EQU CR=10.times.log.sub.10 [IB(off)/IA(off)] (1)
In the conventionally used mechanical optical switching system to change the light path, the switching time is several msec or more. Further, because of its mechanical system, there is involved a problem of wearing properties in the driving part, and this causes deterioration of the reliability required for an optical switching device. On the other hand, the optical switching device using the electrooptic effect of the above-mentioned nematic liquid crystal material works depending on with or without applying a voltage to the nematic liquid crystal material, and it is not driven mechanically. Hence, the optical switching device of this type lasts long. However, the liquid crystal switching device (1') has a problem of long (slow) switching time, e.g., several msec, because the liquid crystal material layer in the liquid crystal cell (11') is composed of a nematic liquid crystal material.
The optical switching device used for changing the light path in the field of optical communication art is described above. Also in the nematic liquid crystal optical switching element used for on-off of the optical signals transmitted on the optical transmission line by means of the open-close operation of the light path, there is involved a problem that the switching time is several msec or more (slow).
Accordingly, an optical switching device capable of realizing a high-speed change of a light path and a high-speed open-close (on-off) operation of a light path has been now eagerly desired.
Next, the technical background of an optical shutter of the invention for use in the field of stereoscopic image display art is described below.
Conventionally used in the field of stereoscopic image display art is a stereoscopic image display method in which images for being observed by each of right and left eyes are formed on an image display screen such as a CRT or a liquid crystal display panel by means of scanning, and then are changed alternately based on each scanning field simultaneously with optical shuttering, and the open-close (on-off) operation of the optical shutter is carried out synchronously with the change of the image so that the stereoscopic image can be observed with time sharing.
As the system for allowing the observer to observe the stereoscopic image with time sharing as mentioned above, there can be used the following two systems.
(a) A system wherein optical shutters, such as spectacles using optical shutters in place of lenses, are positioned between eyes of the observer and the image display screen.
(b) A system wherein an optical shutter for alternately changing the polarization direction of the transmitted light is positioned between the image display screen and the observer so that the image display screen can be observed by the observer through the optical shutter, and by the operation of the optical shutter, the image display screen can be observed with the horizontally polarized light and the vertically polarized light alternately, and spectacles using in place of lenses polarization plates which transmit lights different in the polarization direction, e.g., a horizontally polarized light and a vertically polarized light, are fitted.
Examples of the optical shutters employable for the stereoscopic image display method include a mechanical optical shutter and a non-mechanical optical shutter. They are appropriately used in accordance with the above-described observation system, but an optical shutter utilizing a liquid crystal cell using a nematic liquid crystal material has been paid much attention because it can be used for both the above systems (a) and (b), and can be driven at a low voltage and a low electric power.
In the display of the stereoscopic image, however, the images observed by the right and left each eyes are required to be changed at an interval of not more than about 30 msec, and in order to conduct the open-close (on-off) operation of the optical shutter synchronously with the change of the image, the response time of the optical shutter must be made not more than about 1 msec.
Moreover, the angle range in which the displayed image can be well seen (hereinafter referred to as "angle of visibility") is desired to be wide similarly to the CRT in order to obtain a stereoscopic image of high quality.
In the above-mentioned viewpoints, the optical shutter using a liquid crystal cell having the nematic liquid crystal material layer has various problems, for example, the response time for the open-close (on-off) operation of the optical shutter, namely, open-close (on-off) operation of the light path, is long, e.g., several msec; and when the response time is short, the contrast is reduced or the angle of visibility is narrowed.
As explained above, in the field of the optical communication art and the field of the stereoscopic image display art, accordingly, a light modulation device capable of realizing a high-speed change of a light path and a high-speed open-close (on-off) operation of a light path has been eagerly desired.